1. Field of the Invention
The present invention is generally related to a proximity sensor and, more specifically, to a sensor which utilizes magnetoresistive elements to determine the presence of magnetically permeable material within a predefined detection zone proximate one of the magnetic poles of a permanent magnet.
2. Description of the Prior Art
Many different types of proximity sensors are known to those skilled in the art. Some of these sensors utilize permanent magnets to detect the presence or absence of a magnetically permeable object within a certain predefined detection zone relative to the sensor. In combination with the permanent magnet, some sensors of this type utilize Hall effect elements located at particular positions relative to the permanent magnet and other sensors utilize magnetoresistive elements disposed at advantageous position relative to the permanent magnet. Proximity sensors of this type, whether they use Hall effect elements or magnetoresistive elements, can be configured to sense the presence or absence of a magnetically permeable object passing through a detection zone in a direction generally perpendicular to a central axis of the permanent magnet or, alternatively, can be configured to detect the distance of a magnetically permeable object moving in a direction toward or away from a pole face of the permanent magnet along with a path that is generally parallel to the central axis of the magnet.
U.S. Pat. No. 4,970,463, which issued to Wolf et al on Nov. 13, 1990, describes a temperature stable proximity sensor which senses magnetic flux emanating from the lateral surface of a permanent magnet. The ferrous object sensor is capable of sensing the presence or absence of an object of high magnetic permeability, such as a tooth or a notch on a rotatable mounted ferrous wheel at zero speed and immediately upon power-up. The device can therefore be used as a proximity sensor. It comprises a permanent magnet and a magnetic flux responsive sensor which has a sensing plane and which produces an electrical output signal that varies as a function of the change in magnetic flux density. The ferrous body sensor assembly does not rely upon pole face magnetism as some known conventional sensors do but, instead, relies upon the radial component of magnetic flux density emanating from a lateral surface of the magnet between the opposing pole faces. Since the ferrous object sensor assembly does not rely on pole face magnetism, its electrical output signal is relatively stable over a relatively wide temperature range.
U.S. Pat. No. 4,506,217, which issued to Rothley et al on Mar. 19, 1985, describes a geartooth position and speed sensor with four magnetic resistance tracks connected in a bridge circuit arrangement. To simplify a field plate effect speed and position sensor, four meander-arranged permalloy resistance tracks are located on a substrate at the corners of a rectangle. They are spaced, in the circumferential direction, by approximately half the pitch distance of the teeth of a gear. The resistance can be connected in a voltage divider configuration or in the form of a bridge circuit supplied with a constant current source to eliminate temperature variation effects. In a preferred embodiment, the resistances are formed as meander-shaped thin film vapor deposited tracks on a silicon substrate. A permanent magnet is used to provide bias magnetization.
In proximity sensors of the general type described above, a magnetically sensitive component is generally used to provide a signal representing the strength of a magnetic field in a particular direction. If a Hall effect element is used in association with the permanent magnet, the signal from the Hall element represents the magnetic field strength component in a direction perpendicular to the sensing plane of the Hall device. If, on the other hand, a magnetoresistive element is used in association with the permanent magnet, the signal from the magnetoresistive element represents the magnetic field strength in a direction within the sensing plane of the magnetoresistive element and perpendicular to its thinnest dimension. Depending on the particular application and performance requirements of the sensor, either Hall effect elements or magnetoresistors can be used. Throughout the literature describing the prior art, sensors of this general type are occasionally described as proximity sensors and alternatively described as geartooth sensors, depending on the intended application of the sensor.
In most proximity sensors, several attributes are advantageous. For example, in a geartooth sensor used in association with an internal combustion engine, an advantageous characteristic is the ability to provide a signal upon startup that identifies the presence or absence of a geartooth in a predefined detection zone without the necessity of gear movement. This is known as a power-up recognition capability. Another advantageous characteristic of a geartooth sensor or a proximity sensor is its reduced size. The size of a proximity sensor is usually affected by the size of the permanent magnet and the relative positions of the magnetically sensitive component and the permanent magnet.